1. Field of the Invention
The present invention relates to a heating apparatus for use in an image forming apparatus such as a facsimile machine and a laser beam printer.
2. Description of the Related Art
The image forming apparatus includes a heating apparatus used to heat and fix a toner image transferred to a recording material. The heating apparatus includes a nip portion made of a heating member maintained at a predetermined temperature and a pressure roller pressure-contacted with the heating member. The nip portion uses a process of heating a recording material while pinching and conveying the recording material as a heated material. Particularly, as the heating member of the heating apparatus using a film heating process, there is generally used a heater with a resistance heat generation member formed on a substrate such as a ceramic. When a heater with the same resistance value is used in the heating apparatus using a resistance heat generation member in regions with a commercial power supply of a 100 V system and a 200 V system, the maximum power capable of being supplied to the heater with a commercial power supply of 200 V system is four times that of a 100 V system. This is because the power supplied to the heater is proportional to the square of the voltage. Note that, for example, the commercial power supply of a 100 V system is in a range of a commercial power supply of 100 V to 127 V; and the commercial power supply of a 200 V system is in a range of a commercial power supply of 200 V to 240 V. The larger the maximum power suppliable to the heater, the larger the effects of a harmonic current, flicker, and the like generated by a heater power control, such as a phase control and a wavenumber control. In addition, the power generated when the heating apparatus suffers a run-away phenomenon increases to four times, and thus a more responsive safety circuit is required. Therefore, there is often used a heating apparatus using a heater having different resistance values in a region with a commercial power supply of a 100 V system and in a region with a commercial power supply of a 200 V system. In contrast to this, there has been proposed a unit for implementing a heating apparatus (hereinafter referred to as a universal type of heating apparatus) that can be shared between in a region with a commercial power supply of a 100 V system and in a region with a commercial power supply of a 200 V system. As the unit for implementing the universal-type heating apparatus, for example, there has been proposed a method of switching the heater resistance value using a switch unit such as a relay. For example, Japanese Patent Application Laid-Open No. H07-199702 and U.S. Pat. No. 5,229,577 disclose a heating apparatus having a configuration of a first current path and a second current path extending in a longitudinal direction of the heater (a direction orthogonal to the conveyance direction of a recording material). There is proposed a method of switching the heater resistance value by switching between a first operating state of conducting by serially connecting the first current path and the second current path and a second operating state of conducting by a parallel connection of the first current path and the second current path.
The methods of switching between the serial connection and the parallel connection of the two current paths will be described in detail. Japanese Patent Application Laid-Open No. H07-199702 discloses a method of using a make contact (always-open-contact) relay or a break contact (always-close-contact) relay and a BBM contact (break-before-make contact) relay. Note that instead of the BBM contact relay, two make contact relays or a make contact relay and a break contact relay may be used. U.S. Pat. No. 5,229,577 proposes a method of using two BBM contact relays. According to the above methods, a determination is made as to whether the supply voltage is a 100 V system or a 200 V system; based on the determination, the heater current path is switched between the serial connection and the parallel connection; and thus the heater resistance value can be switched without changing the heating region of the heater.
However, in the aforementioned method (configuration) of switching between the serial connection and the parallel connection, a failure in a supply-voltage detection part or a resistance-value switching relay may cause the heater to enter an overpower-suppliable state. For example, in a state in which a supply voltage of a 200 V system is supplied and in a state in which the heater resistance value is reduced (second operating state), a power four times as large as normal can be supplied to the heater. Therefore, a conventional safety circuit using a temperature detection element such as a thermistor, a temperature fuse, and a thermo switch may suffer from an insufficient response speed. Thus, a heating apparatus capable of switching the resistance value needs to have a unit for detecting a failure state in which large power may be supplied to the heater or a unit for suppressing the power supplied to the heater regardless of the operating state of the heater.